The invention concerns a process for burning minerals for the purpose of deacidifying, sintering or melting, and an installation for carrying out the process.
Hitherto, minerals extracted by surface working, semi-surface working and underground working, such as natural binding agents, iron, copper and silver ores, etc., in grain sizes from about 10 to 200 mm, have been burnt in shaft, circular chamber, rotating or specialized furnaces. This involved a variety of disadvantages, some of them serious:
A rotary furnace for a daily output of about 700 t is up to some 130 m long and, at about 1 rpm, has a diameter of up to 4.0 m. The radiation and heat loss of such a furnace, even when the best heat recovery equipment is used, is around 29%, and a piece of a grain size of about 20 mm requires some 20 to 35 hours to pass through the furnace. A rotary furnace requires between 1350 and 1800 kgcal/kg of burnt product emerging.
In a shaft furnace, larger grain sizes, at least 30 mm, have to be processed, and here, the heat losses today amount to about 21% and the heat consumption lies between 900 and 1200 kgcal/kg. The transit time of a piece is about 32 hours, depending on the raw material and the performance of the type of furnace in question.
The costs of manufacture of existing furnaces are relatively high. A rotary furnace costs up to 30 million francs and a shaft furnace with much the same output about 6 million francs.
All known types of furnace at pressures up to 300 mm water column (corresponding to 0.3 atu) and a grain size of 20 to 200 mm. However, as every specialist knows, limestone forms a calcined skin on its surface during burning, which has a highly heat-insulating effect and allows heat to penetrate into the interior of the stone only with difficulty. The burnt skin hinders or slows down the solid body reactions. In consequence, the stone has to be exposed to the prolonged action of heat until it is completely burnt through; by then, however, the outer skin is usually damaged (overburnt) or it breaks down into a fine powder which clogs the furnace, settles on the other stones and insulates them, as long as the furnace is in operation.
Rotary and shaft furnaces also consume an exceptionally large amount of power and, even with maximum automation, require at least one man per shift. In addition, it is impossible to start up such a furnace within a shorter period than 3 days, and it is also not easy to shut it down again. Costs are extremely high and starting up usually calls for special expenditure. Furthermore, a conventional furnace, of whatever design, can be used for one or two binding agents only; its location is fixed and it weighs about 1500 t, often more.
Finally, every known type of furnace can be used only for a specific grain size, with the result that, in every factory, a certain proportion of much finer grain occurs in the waste. This fine grain can be used for only very few other purposes and therefore realises only a very low price (what are known as give-away prices). In the majority of plants, this material lies around in vast quantities and clutters up the limited space, although it is, basically, the best raw material.